openPrefetchingReadChannel is not working in Google Cloud Storage Client API

问题

I am trying to fetch object from bucket using openPrefetchingReadChannel GCSInputChannel. As Google developer tutorial says:

GcsInputChannel readChannel = gcsService.openPrefetchingReadChannel(fileName, 0, 1024 * 1024);

Prefetching provides is a major performance advantage for most applications, because it 
allows processing part of the file while more data is being downloaded in the background 
in parallel.The third parameter is the size of the prefetch buffer, set in the example 
to 1 megabyte.

Well this is not happening for me. Please have a look at my snippet:

  GcsInputChannel readChannel = gcsService.openPrefetchingReadChannel(fileName, 0, 1024);
  copy(Channels.newInputStream(readChannel), resp.getOutputStream());

   private void copy(InputStream input, OutputStream output) throws IOException {
    try {
      byte[] buffer = new byte[BUFFER_SIZE];
      int bytesRead = input.read(buffer);
      while (bytesRead != -1) {
        output.write(buffer, 0, bytesRead);
        bytesRead = input.read(buffer);
      }
    } finally {
      input.close();
      output.close();
    }
  }

Ref: https://code.google.com/p/appengine-gcs-client/source/browse/trunk/java/example/src/com/google/appengine/demos/GcsExampleServlet.java

Above code should deliver 1KB of data from uploaded object but it is returning the whole data of object i.e. 8.4KB. Please look at the screenshot:

I am not sure what is happening. Need your help guys

回答1:

The third argument for openPrefetchingReadChannel is not the max size to read (or limit). Is the the internal buffer size for prefetching. In your case you may want to track how much you read and keep writing until reached the desired limit

来源：https://stackoverflow.com/questions/25704937/openprefetchingreadchannel-is-not-working-in-google-cloud-storage-client-api

标签

java

google-app-engine

google-cloud-storage